BANGKOK (AP) — In the aftermath of a coordinated wave of bombings that shook tourist towns in Thailand this week, Prime Minister Prayuth Chan-ocha’s military government is scrambling to hunt down those responsible. The 11 explosions killed four people and injured dozens, including 11 foreign tourists.

The big questions now are, who did it, and why?

There has been no claim of responsibility, but there are plenty of groups unhappy with the political situation in the Southeast Asian nation. Prayuth came to power in a 2014 coup, and his junta faces opposition from political opponents and activist groups, as well as a long-running insurgency in the nation’s largely Muslim south that has left well over 5,000 people dead since 2004.

A look at groups the government is likely investigating in the latest violence, or has ruled out:

FOREIGN MILITANTS

Thai officials say they don’t believe international Islamic militant groups are responsible, calling the bombings “local acts of sabotage.” The timing of the attacks gives credence to the theory that domestic dissidents were behind it: The explosions occurred several days after Thais approved a new constitution in a referendum that critics say will ensure the military’s hand in politics for the foreseeable future.

Analysts say it was also no coincidence the attacks came on the eve and 84th birthday of Queen Sirikit. The army sees itself as the primary defender of the monarchy and has made clear that protecting it is a top priority. The king and queen have a large palace in the seaside resort of Hua Hin, where some of the bombings took place, and targeting that city has “symbolic implications,” said Pongphisoot Busbarat, research affiliate at the University of Sydney’s Southeast Asia Centre.

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POLITICAL OPPONENTS

Thailand’s military government will undoubtedly probe its primary political rivals, namely the opposition Pheu Thai party it ousted in a 2014 coup and their “Red Shirt” supporters in the north and northeast. A decade ago, in 2006, the army had also toppled their leader, Thaksin Shinawatra, triggering a decade of sporadic upheaval. The Pheu Thai party and its allies have denied allegations of involvement in low-level violence over the past decade, including grenade attacks and this week’s bombings.

Anthony Davis, a writer for Jane’s Defence Weekly, told The Associated Press that it’s “difficult to give much credibility to the suggestion that this could be the work of disgruntled Red Shirt elements loyal to … Thaksin Shinawatra. The Red Shirt movement as a whole has been under extremely tight military monitoring both before and since the military coup of 2014. The theory that they could have organized such a complex operation under the noses of the military government makes no sense.”

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STUDENT AND CIVIL ACTIVISTS

Student and civil society activists have been among the most vocal against the junta, which has clamped down hard on critics, regularly ferrying those who speak out to military camps for “attitude adjustment” sessions. The activists have staged small protests sporadically since the 2014 coup, but the demonstrations have been overwhelmingly peaceful, and few believe they have the capability or desire to instigate violence.

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SOUTHERN INSURGENTS

Ethnic Malay militants fighting for greater autonomy in Thailand’s far south have launched attacks there nearly every day for a more than a decade. Analysts say they are the only factions that have successfully staged sophisticated, coordinated assaults with improvised, remote-controlled explosive devices. Although their targets have overwhelmingly been confined to Thailand’s three southernmost provinces, the militants have apparently carried out isolated attacks elsewhere — detonating, for example, a car bomb in the underground parking lot of a mall on the tourist island of Koh Samui in April 2015 that wounded at least seven people.

Thai police have not singled the militants out yet, but police spokesman Col. Krisana Patanacharoen said Friday that the latest bombings followed “a similar pattern used in the southern parts of the country.”

Davis, the Jane’s Defence Weekly writer, said the militants were the only “non-state actor in Thailand with the capability for a well-planned, well-coordinated operation like this.” Don Pathan, a security analyst based in southern Thailand, said that the latest attacks didn’t seem to fit the militants’ traditional pattern of operations, but that if they were responsible, “it would definitely be a game changer” that could herald a new chapter in the conflict.

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THE MILITARY ITSELF

Conspiracy theories are part and parcel of whodunits in Thailand, and a few have speculated the army or a faction within it might have planted the bombs as part of an internal dispute or a bid to justify military rule by showing threats remain. Asked by a reporter whether the attacks could have been an internal job, a junta spokesman dismissed the idea outright. Writing in the Nikkei Asian Review, Thitinan Pongsudhirak, director of the Institute of Security and International Studies at Thailand’s Chulalongkorn University, said that “Thailand’s governing generals have gained a limited mandate from the referendum, and would be unlikely to undermine their legitimacy by resorting to terrorism of this kind, whatever the skeptics may say.”